Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, alternative energy generation, satellites and electric vehicles.
Modern flywheel energy storage systems convert back and forth between the rotational energy of a spinning flywheel and electrical energy. A flywheel energy storage system includes a flywheel, a motor and generator, a bearing system and usually a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor and generator converts between electrical and mechanical while the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low-pressure enclosure to minimize aerodynamic losses that would occur from operation in a gas at atmospheric pressure.
The vacuum surrounding a flywheel is usually maintained at a pressure of 10xe2x88x921 Torr or lower to significantly reduce drag and the resulting potential overheating of the flywheel. Lower pressures are typically used for composite flywheels, which operate at higher tip speeds and have lower thermal conductivities. When composite flywheels are used, they outgas a large amount in vacuum due to the large volume of epoxy or polymer matrix material. Compared with composite flywheels, systems that employ steel flywheels outgas up to 1000 times less, making maintenance of the vacuum in the enclosure much easier. Despite the lower outgassing with use of a metal, or alternatively a specially constructed or treated composite flywheel, other components of the flywheel system then become the most significant outgas sources. The primary such source is the motor/generator of the flywheel system.
The motor/generator is necessarily constructed using polymer or other type electrical insulation materials that outgas appreciably in vacuum. These outgas sources can include insulation on the motor/generator coils, lamination insulation, epoxy potting and the trapped gasses between lamination surfaces if employed. Compared with a composite flywheel, the amount of these materials in the motor/generator is typically more than 1000 times less. Such a limited outgas source could be considered insignificant and in most cases can be easily pumped away. However, if a getter is used to maintain the vacuum in the flywheel system instead of a vacuum pump, this outgassing can be significant. Getters offer simpler operation and less maintenance than mechanical pumps. Unfortunately, they can only pump a limited amount of gas before becoming used up. Larger molecules such as hydrocarbons, which outgas from polymers, are also not very well sorbed by available getters. Motor/generators also get hot during charging and discharging of the flywheel system, and outgassing from a material is an exponential function of the temperature. Outgassing from the motor/generator can shorten the life of getter vacuum maintained flywheel systems or alternatively, it can require the use of more getter material, increasing costs.
The invention provides a low outgas motor/generator for use in flywheel energy storage systems that employ a nonevaporable getter to maintain the internal vacuum. It is particularly well suited for flywheel systems that use metal or steel flywheels. The flywheel systems in accordance with the invention have a low outgas barrier, typically a metal, on the stator portion of the motor/generator. The coating can cover the electromagnetic coils and their insulation and epoxy potting. It can also enclose any stator laminations, reducing outgassing from both the insulation materials and the gases trapped between laminations. The barrier can be a thin layer coating of metal that reduces the outgassing from the motor/generator stator. In one embodiment, the barrier is a foil such as aluminum or stainless steel foil that is bonded to the surface of the stator. The foil can be cut into pieces and bonded in sections to cover contoured surfaces if easier. In another embodiment, the stator is epoxy potted into the foil and the foil is used to line the potting mold. The use of a thin foil or sheet of metal layer enclosing some or part of the stator the does not cause appreciable losses due to eddy currents induced in the foil, and any heat generated in the foil is also thermally dissipated to the stator. The foil can be made very thin because outgassing is drastically reduced with very thin layers. Use of steel foils or other higher electrical resistivity materials reduces eddy currents and losses even more.
In another embodiment of the invention, the metal coating is applied to the stator by physical vapor deposition. This produces a much thinner layer of only several thousand Angstroms but still very effective and potentially less laborious. PVD can produce both metal coatings such as aluminum and also ceramic coatings. Either type that reduces the outgassing can be used with the invention however aluminum coatings are preferred for ease and low cost. In another embodiment of the invention, PVD can be conducted insitu inside the assembled and evacuated flywheel container. The metal is evaporated and deposited over the motor/generator as well as other components, which can include the flywheel. A shield is used to prevent coating of the nonevaporable getter material during physical vapor deposition.
In another embodiment of the invention, the barrier coating is applied by dipping brushing, spraying or wiping on a coating. A colloidal dispersion of low outgassing particles can be applied to the stator and baked for reduction of outgassing. A dispersion of fine graphite particles, sold under the name trade name Aquadag can be used. Tests on outgas reduction from coating polymers has shown that metal foil coatings reduce outgassing by a factor of 20, physical vapor deposited aluminum by a factor of 10 and carbon particle coating by a factor of 2-6. These outgas reductions can extend the life of the flywheel system and can reduce the amount of getter material required.